Reverse Osmosis Vs Nanofiltration Systems

Membrane Filtration is the separation or removal of particulate and colloidal particles from a liquid. The process is facilitated across a physical barrier (semipermeable membrane) which restricts the passage of particles by their finely sized pores.

The water entering the membrane module is called the ‘Feed Stream’.

The liquid that passes through the semipermeable membrane is called the ‘Permeate or Product Stream’.

The liquid containing the retained particles on the other end of the membrane is called the ‘Retentate/Concentrate/Reject/Waste Stream’.

Types of Membrane Filtration Processes which are more commonly used today are based on the pressure difference between the two sides of the membrane. These include:

1) Microfiltration (MF)
2) Ultrafiltration (UF)
3) Nanofiltration (NF)
4) Reverse Osmosis (RO)
[custom-specifications]

Reverse osmosis vs nanofiltration is a hotly debated subject in the water treatment industry. Supporters of both membrane filtration technology have continued to highlight the advantages of each water purification system that makes them the better of the two. RO and NF systems share similarities in terms of appearance and features. However, nanofiltration systems utilize newer technology and incorporates added features which allow it to do certain things that RO systems can not perform as efficiently.

For instance, reverse osmosis systems were found to filter out precious nutrients that are beneficial for human consumption. Nanofiltration systems were designed to ensure this does not happen, including being much more capable than RO in removing bivalent ions that create hardness in water. Reverse osmosis has been utilized for a greater time than NF and continues to be the most preferred choice in applications that require the removal of the smallest solute molecules such as salt and various monovalent minerals.
[/custom-specifications]

[custom-features]

Industrial Applications:

With the development of thin-film and composite membranes, the use of reverse osmosis technology expanded from the traditional desalination process to a wide range of multiple wastewater treatment applications. The application for RO systems includes the treatment of organic-containing wastewater, wastewater from electroplating, metal finishing, pulp and paper, mining and petrochemical, textile, food processing industries, radioactive wastewater, irrigation water treatment, municipal wastewater, surface water treatment and contaminated groundwater, especially for the removal of N-Nitrosodimethylamine (NDMA) and other organic disinfectant by-product precursors (DBPs).

Nanofiltration has caught the attention of many for the purpose of water softening and the removal of various contaminants from drinking water. NF can reduce or remove TDS, hardness, color, agricultural chemicals, high molecular-weight, fulvic materials (which can form trihalomethanes when chlorinated). NF is also used to remove both organics and inorganics from different wastewaters. NF is an efficient water treatment system for secondary or tertiary effluents for the purpose of industrial, agricultural, or indirect potable reuse.

Conclusion:

Membrane separation can clearly speak for itself when compared to other water treatment solutions. Although the conceptual process of nanofiltration and reverse osmosis maybe similar, they each have distinctive factors and usages that make them unique. Nanofiltration requires less energy and produces higher fluxes at low pressures; works best for applications that do not require a feed stream completely free of dissolved solids. Reverse Osmosis, on the other hand, is the solution for absolute purified water although it requires more energy to produce.
[/custom-features]

[custom-usage]

The various membrane filtration processes can be further classified and compared by the following categories:

1.) Driving Force
2.) Removal Mechanism
3.) Operating Principle
4.) Industrial Applications

Here, we are going to focus on Reverse Osmosis (RO) and Nanofiltration (NF); which are two of the most commonly used types of membrane filtration. We shall now look into the two systems (NF and RO) closely, based on the categories listed above.

1.) Driving Force:

Both nanofiltration and reverse osmosis systems are carried out on the basis of hydraulic pressure to bring about the desired separation i.e., the transfer of particles through the semipermeable membrane.

2.) Removal Mechanism:

Unlike Microfiltration (MF) and Ultrafiltration (UF), in which the separation process depends only on size exclusion (sieving); NF and RO not just provide separation based on size but also provide the adsorption of ionic species from the water layer adsorbed on the membrane surface.

3.) Operating Principle:

Reverse osmosis is a process of filtration through applying pressure on water to pass through a membrane and remove particles and dissolved solids. Water is forced to flow from the concentrated to the diluted side, and solutes are retained by the filtration process.

Nanofiltration, frequently described as a looser version of RO, uses fewer fine membranes compared to a RO system. This is shown as the feed pressure required becomes almost half of what is necessary in a RO system.

There is a considerable overlap in the sizes of particles removed by NF and RO. Typically, nanofiltration systems can reject particles as small as 0.001 μm whereas reverse osmosis systems can reject particles of size up to 0.0001 μm.
[/custom-usage]